Sensing elements and method for making same

Abstract: Sensing elements, useful in sensing the concentration of a gas, e.g., carbon dioxide, in a medium, e.g. blood, and methods for making such sensing elements are disclosed. In one embodiment, the method comprises continuously placing a sensing composition precursor in contact with a continuous web, forming a continuous sensing composition from the precursor, and applying a continuous opaque film to the sensing composition. A plurality of individual sensing elements are formed from this composite structure. These sensing elements, which each have very similar sensing characteristics, are placed in a sensor fixture for use.

Description: This invention is related to sensing elements useful in sensing an analyte of interest, and to methods for making such sensing elements. More particularly, the invention relates to sensing elements and to methods for "mass producing" sensing elements, for example, useful in sensing the concentration of a gas, such as, carbon dioxide or oxygen, in an aqueous medium, for example, blood. In many situations, it is useful to determine the concentration, e.g. partial pressure, of a gas in a medium, e.g., a fluid medium. One such situation is the determination of the concentration of gas, e.g., carbon dioxide or oxygen, in blood. Fluorescence-based sensors have been utilized to accomplish real time blood gas sensing. For example, fluorescence-based sensors can be used in an extracorporeal blood loop as shown in Cooper U.S. Patent No. 4,640,820 and in vivo as disclosed in Lubbers et al., Reissue U.S. Patent No. 31,879. EP-A-0 448 052 discloses a method for making a sensing element, and an assembled sensing element, useful in sensing the concentration of a gas, such as carbon dioxide, in a medium such as blood. In one embodiment, the method includes the steps of placing a quantity of a sensing composition precursor between and in contact with a transparent, gas impermeable solid disc and an opaque, gas permeable film. The sensing composition precursor comprises a sensing component, a polymer precursor and a cross-linking agent. WO 92/15880 refers to a process for manufacturing a self-supporting material for a test field. In the process, a reagent film material is pressed from an application side into a web carrier material, which consists of an open-pored composite fiber structure, so that it thoroughly penetrates the pores of the carrier material. The reagent film material is spread smoothly over the opposite side by the relative motion of the web and a smoothing tool, so that the pores of the fibrous composite structure are covered over by the reagent film material. Finally, the web of test field material is dried essentially free of contact. Yafuso et al., U.S. Patent 4,824,789 disclose a sensing composition and sensor useful for determining the concentration of a gas in blood. This patent discloses a sensing composition which is useful when placed on the optical surface of an optical fiber to form a sensor. The sensing composition comprises an aqueous first phase dispersed in a cross-linked polymeric second phase. The aqueous first phase includes a dye, for example a pH sensitive dye. The cross-linked polymeric second phase, which is gas permeable and ion impermeable, preferably is silicone-based and includes a hydrophobic filler, such as hydrophobic fumed silica particles. These sensing compositions may further include one or more emulsification enhancement agents, such as water soluble dextran and polyvinyl alcohol. This patent discloses that a teflon sleeve is used on the optical fiber during manufacture of the sensor to retain the sensing composition precursor on the optical surface of the optical fiber. The sensor also includes an opaque overcoat, such as cellulose impregnated with carbon black, to optically isolate the sensing composition. The use of disposable cassettes for blood analysis is of substantial current interest, for example, to eliminate cross-patient contamination and to keep more expensive components, e.g., optical and electronic components, of the sensor system from being exposed directly to blood. One disposable cassette system is disclosed in U.S. Patent 4,989,606. Such a disposable cassette includes an indent, well or cavity into which a sensing element is at least partially placed. The sensing element is exposed to blood and the sensing component, e.g., a fluorescent dye, gives off a signal which varies in response to variations in the concentration of the gas of interest in the blood. A signal transmitter, e.g., an optical fiber, spaced away from the cassette well and blood transmits this signal to a processor where it is analyzed to provide the desired blood gas concentration determination. One problem which has presented itself with regard to the use of these cassettes is that the characteristics of the sensing elements vary significantly from one to another. Also, when the sensing element is manufactured or assembled in situ , for example, in the well of the cassette, there is no chance to screen the sensing element before it is secured to, for example, bonded to, the cassette. Thus, out-of-specification sensing elements can be included in the cassette and determined as being non-conforming only after the assembled cassette, which often includes a number of other sensing elements for different blood constituents, is tested. At this point, the entire cassette must be discarded, resulting in substantial waste and cost. It would be clearly advantageous to provide new sensing elements and/or new methods for making sensing elements, particularly for use in such cassettes. New sensing elements, and methods for making sensing elements, useful in sensing an analyte, such as a gas, for example, carbon dioxide or oxygen, in a medium, for example, blood, have been discovered. The present sensing elements provide substantial benefits. For example, these elements can be produced so as to have substantially uniform characteristics, and then tested or screened to assure conformity to product specifications, before they are included in the final sensor holder or fixture. This feature reduces the cost and time required to produce high quality sensors which provide reliable and consistent analyte sensing results. In addition, the present sensing elements are preferably produced in a form which is very convenient to use when including individual sensing elements into sensor holders or fixtures. The present methods for producing sensing elements are straightforward, relatively easy to practice, and very effectively mass produce many sensing elements which have very similar, if not identical, sensing characteristics. The relative amount of out-of-specification sensing elements produced is reduced and, since the sensing elements can be tested or screened before being included with the other sensor components, the cost of producing out-of-specification sensing elements is even further reduced. Moreover, because of the substantial uniformity achieved in mass producing the present sensing elements, the amount of testing of sensing elements can be greatly reduced, thereby even further reducing costs without adversely impacting the quality of the final sensing elements. In one broad aspect of the present invention, methods for making sensors adapted for sensing an analyte are provided. In particular, the present invention provides a method for making a sensor capable of sensing an analyte, which method comprises: continuously placing a sensing composition precursor on a continuous web thereby forming a continuous precursor-containing web; forming a continuous sensing composition responsive to the analyte from said sensing composition precursor included in said continuous precursor-containing web, thereby producing a sensing composition-containing web, said continuous sensing composition being sized and structured to be dividable amoung a plurality of sensing elements; and wherein said forming step includes removing said continuous web from said sensing composition-containing web. Preferably said method comprises: placing a sensing composition precursor on a continuous web, thereby forming a continuous precursor-containing web; forming a continuous sensing composition responsive to the analyte from said sensing composition precursor included in said continuous precursor-containing web, thereby providing a sensing composition-containing web, said continuous sensing composition being sized and structured to be dividable among a plurality of sensing elements; and wherein said sensing composition comprises: an aqueous first phase comprising a pH sensitive indicator component which is effective to provide a signal in response to the concentration of analyte in a medium to which said sensing composition is exposed, said analyte being effective to alter the pH of said primary phase; and a hydrophobic second phase which is permeable to said analyte and impermeable to ionized hydrogen, wherein said hydrophobic phase lies between said aqueous phase and said medium to which said sensing composition is exposed, wherein said sensing composition is substantially free of partitioning species other than the analyte of interest which can migrate from one phase to the other in response to a change in pH in said aqueous phase and which substantially affect said concentration dependent signal. In another embodiment the present invention refers to a method for making a sensor capable of sensing an analyte, which method comprises: continuously placing a sensing composition precursor on a continuous web thereby forming a continuous precursor-containing web; forming a continuous sensing composition responsive to the analyte from said sensing composition precursor included in said continuous precursor-containing web, thereby producing a sensing composition-conteining web, said continuous sensing composition being sized and structured to be dividable among a plurality of sensing elements; and forming a plurality of sensing elements from said sensing composition-containing web, each of said plurality of sensing elements including a sensing composition layer, and securing an individual sensing element formed from said sensing composition-containing web to a sensor fixture; wherein each of said plurality of sensing elements includes no portion of said continuous web. Sensors for measuring the concentration of an analyte in a medium are also described. The continuous web preferably is made of a polymeric material. In one embodiment, the sensing composition precursor comprises a sensing component, which is responsive to the analyte of interest, and a polymer precursor. The above-noted forming step preferably involves polymerizing this polymer precursor, such as by employing a photo activatable, for example, an ultraviolet light activatable, polymerization catalyst component. The sensing composition is preferably useful in sensing the concentration of a gas, for example, carbon dioxide or oxygen, in a medium, for example, an aqueous medium such as blood. Continuously placing the sensing composition precursor on the continuous web provides substantial advantages. For example, it has been found that a substantially uniform coating of sensing composition precursor can be continuously placed on the web much more easily, controllably and reliably relative to placing the precursor on the web in a batch-type process, such as by coating relatively small individual sheets of the web material. Moreover, since very large numbers of individual sensing elements can be produced in a single "run" using this continuous processing, the almost inherent and disadvantageous variability from individual sheet to sheet obtained with batch-type processing is avoided. Also, adjustments can easily be made during the continuous processing, particularly early in the run, to provide that a sensing composition precursor/web of desired quality is achieved. The continuous web is preferably treated or coated to enhance the adhesion of the sensing composition to the continuous web, for example, relative to the adhesion of the sensing composition to an identical continuous web which has not been so treated or coated. Such treatment may include one or more of the following: subjecting the continuous web to a plasma treatment, and subjecting the continuous web to a corona discharge treatment. The adhesion enhancing coating on the continuous web may comprise a primer agent or the like material. In a further particularly useful embodiment, the present methods include forming or providing a continuous opaque film on the sensing composition, for example, so that the sensing composition is located between the continuous web and the opaque film. This opaque film can be formed prior to being associated with the sensing composition or it can be formed in situ on the sensing composition. The continuous opaque film is permeable to the analyte to be sensed. Such an opaque film is particularly useful in applications where the final sensing element derived from the continuous web/sensing composition composite structure is to be optically isolated, for example, to reduce reflection and/or other optical interferences so that more consistent sensing signals are provided and, ultimately, to promote or facilitate the accuracy of the sensor. In one embodiment, the sensing composition, noted above, can be a continuous adhesive member. Once the sensing composition-containing continuous web is produced, a plurality of individual sensing elements can be formed, for example, cut, from this material. Each of the plurality of sensing elements includes a sensing composition layer (derived from the continuous sensing composition). the sensing elements do not include a portion of the continuous web. For example, after the sensing composition-containing web is formed, the web can be physically stripped away, dissolved or otherwise removed from the sensing composition so that the sensing elements that are ultimately formed do not include a web portion or layer. Once the individual sensing elements are formed, each of such elements can be used by being secured, preferably adhesively secured, to a sensor holder or fixture, for example, in a cavity or indent of a sensor cassette. Sensing composition precursors, for example, including light activat able polymerization catalyst components as described herein, are within the scope of the present invention. Sensing composition-containing webs comprise a continuous web, and a continuous sensing composition responsive to the analyte to be sensed and located on the continuous web. The continuous web and continuous sensing composition are as described elsewhere herein. The present sensing composition-containing webs can be produced in accordance with the methods described herein. In addition, sources of individual sensing elements, for example, including continuous release membranes, and individual sensing elements, are described elsewhere herein. After the individual sensing element has been produced, and it has been determined to meet product specifications, it is secured to a sensor holder or fixture, preferably at least partially in a cavity or well having an open end located in a sensor holder. The sensing element is preferably oriented so that the sensing composition is exposed to the medium being monitored. The sensor, comprising the sensing element, and sensor holder or fixture, is now ready for use in a sensing application, for example, monitoring the concentration of a gas in blood, as desired. These and other aspects of the present invention are set forth in the following detailed description and claims, particularly when considered in conjunction with the accompanying drawings in which like parts bear like reference numerals. Fig. 1 is a block diagram illustrating two methods of producing sensors. Fig. 2 is a cross sectional view showing an assembled sensing element being bonded to a sensor cassette. Fig. 3 is a schematic view showing a sensing element bonded to the sensor cassette for providing signals useful to sense the concentration of a gas in a medium. Fig. 4 is a cross sectional view showing an assembled sensing element being bonded to a sensor cassette. Fig. 5 is a partial top front view, in perspective, of a composite structure from which individual sensing elements are produced. Fig. 6 is a top front view, in perspective, of an individual sensing element formed from the composite structure shown in Fig. 5. Fig. 7 is a partial top front view, in perspective, of a composite structure from which individual sensing elements are produced. This drawing shows a number of the composite layers broken away for illustrative clarity and, in addition, shows an individual sensing element produced from the composite structure. In the drawings, no attempt has been made to present the individual layers of the composites and sensing elements as having different thicknesses. It should be understood, however, that such differences do exist, as described herein. In one important aspect, the present invention is directed to methods for producing sensors including individual sensing elements, such as individual gas sensing elements, for example, carbon dioxide sensing elements or oxygen sensing elements, useful in sensing an analyte, such as a gas, in a fluid medium, such as blood. The present methods of producing sensors are described with particular reference to Fig. 1, and the layered composites, composite structures and individual sensing elements formed are described with particular reference to Figs. 5, 6 and 7. The methods for producing sensors illustrated in Fig. 1 comprise a series of steps, each step being represented by a different box in Fig. 1. Each of these steps is discussed in detail hereinafter. Briefly, however, these methods involve the following steps. In an initial portion of the methods, shown generally at 2 in Fig. 1, a sensing composition precursor is formed and placed, for example, coated, on a continuous web. A continuous sensing composition is formed from the precursor and is sized and structured to be dividable among a plurality of individual sensing elements. The sensing composition precursor often includes a polymer precursor which is cured, that is polymerized and/or cross-linked, in forming the continuous sensing composition. A continuous opaque film is placed, for example, applied or formed, so that this film is located on the continuous sensing composition, and the sensing composition is positioned between the opaque film and the continuous web so that a continuous web/sensing composition/opaque film composite is formed. This composite is illustrated at 8 in Fig. 5. The individual sensing elements can be formed from composite 8, preferably using one of two alternate approaches. In one method illustrated generally at 4 in Fig. 1, the individual sensing elements are formed from the continuous web/sensing composition/opaque film composite 8. This forming may involve cutting or otherwise dividing the composite 8 to produce the individual sensing elements 10 (cf. Figure 6; not according to the invention) each of which has a web layer 12 (derived from the continuous web), a sensing composition layer 14 (derived from the continuous sensing composition in the composite) and an opaque layer 16 (derived from the continuous opaque film). After testing and/or screening to determine that these individual sensing elements are satisfactory, for example, meet product specifications, these elements are secured to sensor holders or fixtures, for example, cavities or indents in sensor cassettes. The individual sensing elements, positioned as noted above, are ready for use. In another method not according to the invention illustrated generally at 6 in Fig. 1, an adhesive composition layer, including an adhesive composition, preferably comprising a pressure sensitive adhesive composition (PSA), is placed on the continuous web of the web/sensing composition/opaque film composite. This adhesive layer, which is located so that the continuous web is between the adhesive layer and the continuous sensing composition, is preferably located on a release liner. Thus, an additional composite, illustrated generally at 20 in Fig. 7, is formed in which the adhesive layer is located between the release liner and the continuous web. Thus, additional composite 20 includes release liner 22, adhesive layer 24, continuous web 26, continuous sensing composition 28 and continuous opaque film 30. This additional composite 20 is "converted" to form individual sensing elements 32 located on the release liner 22. This "converting" or "individual sensing element forming" step is accomplished by cutting or otherwise dividing the additional composite 20 into individual sensing elements 32 located on the release liner 22, which remains intact during this step. Each of these individual sensing elements 32 includes an adhesive composition layer 34 (derived from adhesive layer 24), a web layer 36 (derived from continuous web 26), a sensing composition layer 38 (derived from continuous sensing composition 28) and an opaque layer 40 (derived from continuous opaque film 30). Material 42 which is not included in any of the sensing elements 32 is removed from the release liner 22. The remaining structure, that is the release liner 22 carrying individual sensing elements 32, is a very convenient source of individual sensing elements. Thus, after appropriate testing and/or screening to determine that the individual sensing elements 32 are of acceptable quality, they can be individually removed from the release liner 22 and secured to a sensor holder or fixture, for example, a cavity or indent in a sensor cassette, so that the adhesive layer is in contact with the sensor holder or fixture. In this manner, the individual sensing element is adhesively secured to the sensor holder or fixture and is ready for use. In more detail now, the present methods, and composites, sensing elements, and materials employed therein are as follows. A sensing composition precursor is formed or otherwise provided. Such precursor is useful to be applied as a continuous, preferably as a substantially uniform continuous, coating on the continuous web and, while on the web, to be reacted or otherwise converted or transformed into the presently useful continuous sensing composition. The sensing composition, and preferably the sensing composition precursor, includes an effective amount of a sensing component which is responsive to, and provides signals indicative of, the analyte of interest. The sensing composition precursor is preferably such that the resulting sensing composition is permeable to the analyte of interest. Although any suitable sensing composition precursor may be employed, in a particularly useful embodiment the precursor comprises a sensing component and a polymer precursor, that is a precursor of a polymeric material which is part of the sensing composition. The polymeric material of the sensing composition is permeable to the analyte, preferably gas, of interest in the medium being monitored, and is permeable or transparent to the wave lengths of light utilized when, as is preferred, optical signals are used in sensing the analyte. Further, if the sensing composition is to be used in sensing carbon dioxide, this polymeric material is preferably impermeable to ions and to liquid water. For example, if the sensing component is dissolved in a dispersed aqueous liquid and, as is ' preferred, the aqueous liquid contains a buffer, the concentration of the buffer ions is preferably maintained substantially constant so that the sensing composition provides consistent signals in response to the concentration of the analyte of interest, carbon dioxide, in the medium. Any suitable polymeric material may be employed in the present gas sensing compositions provided that the polymeric material has no substantial detrimental effect on the functioning of the present system or on the medium being monitored. Because of substantial gas and light permeability and aqueous impermeability properties, silicone-based polymeric materials are preferred. More preferably, cross-linked silicone-based polymeric materials are employed. When the analyte of interest is carbon dioxide or oxygen, the polymeric material is preferably cross-linked polydimethyl siloxane or copolymers of polydimethyl siloxane. The precursor of the polymeric material, the polymer precursor, may be selected from one or more monomers, pre-polymers, and mixtures thereof. In one embodiment, the polymer precursor is a precursor of a vinyl/hydride addition cure polysiloxane polymer. A particularly useful polymer precursor, e.g., when carbon dioxide or oxygen is the analyte of interest, is vinyl terminated dimethyl siloxane. If the polymeric material is to be cross-linked, a cross-linking agent is included with the polymer precursor. Such cross-linking agents are preferably compounds which include at least three functional groups per molecule capable of reacting with the polymer precursor and/or a partially polymerized intermediate to form cross links, e.g., between polymer chains, in the polymeric material. A particularly useful cross-linking agent is methylhydro-dimethylsiloxane copolymer especially when the polymer precursor includes vinyl terminated dimethylsiloxane. One or more catalysts may be used to promote the formation, by polymerization, of the polymeric material. One such catalyst is platinum. The sensing composition precursor, including a catalyst component, may be exposed or subjected to elevated temperature sufficient to facilitate the catalyst component in promoting polymerization. Such elevated temperatures are greater than ambient temperature (22° C) and preferably are in the range of about 40° C to about 100° C or more. The amount of catalyst component used should be sufficient to promote the desired degree of polymerization. Of course, the catalyst component should have no substantial detrimental effect on the functioning of the present system or on the medium being monitored. As used herein, the terms "polymerization" and "polymerizing" relate to one or more chemical reactions, including cross-linking, by which a polymer or polymeric material is formed. In a useful embodiment, the catalyst component is chosen so as to be activatable (for example, to promote polymerization of the polymer precursor), such as photo activatable, upon being exposed to one or more factors, such as to light energy, preferably selected from visible light energy and ultraviolet light energy, and/or elevated temperatures, as defined herein, and the like. One substantial advantage of such a "factor" activatable catalyst is that the polymer precursor, including the catalyst, can be prepared and maintained as such for relatively long periods of time by avoiding exposing the precursor to the "activating factor". Thus, one important process variable, that is the time when the polymer precursor is polymerized, in producing the present sensors is controlled so that the need to follow a tight production schedule because of the spontaneous polymerization of the polymer precursor is advantageously reduced. When it is desired to polymerize the polymer precursor, the precursor is simply exposed to the "activating factor". A particularly useful class of "factor activated" catalysts are those which are activatable by light energy and, after being activated by sufficient light energy, are effective to partially polymerize the polymer precursor and are also effective upon being exposed to elevated temperatures to promote further polymerization of the partially polymerized polymer precursor. Any suitable "factor" activatable catalyst may be employed in the present invention, provided such catalyst has no substantial detrimental effect on the present system or on the medium being monitored. Photo-activatable hydrosilylation catalyst systems are particularly useful. Certain platinum group metal-containing materials are very effective "factor activatable" catalysts for use in the present invention. A number of such catalyst components are disclosed in Drahnak U.S. Patents 4,510,094 and 4,530,879 and 4,600,484, and in Boardman et al U.S. Patent 4,916,169. Specific examples include cyclopentadienyl trimethyl platinum, derivatives thereof and mixtures thereof, particularly those which, after being activated by visible light energy or ultraviolet light energy, provide additional effective catalytic activity at elevated temperatures. The use of a combination of light energy and elevated temperatures to facilitate polymerization of the polymer precursor allows the polymerization to occur more rapidly and/or more completely relative to systems in which only light energy is employed. Alternately, the silicone-based polymeric material can be formed through condensation polymerization reactions with silanol terminated silicones being cross-linked with alkoxyl silanes using catalysts, such as tin derivatives. Fillers can be, and preferably are, included in the present gas sensing compositions which include dispersed aqueous liquid. Such fillers act to enhance the stability of the dispersed aqueous liquid in the sensing composition and the strength of the sensing composition. Any suitable filler may be used provided it has no substantial detrimental effect on the functioning of the present system or on the medium being monitored. In one embodiment, the filler has a hydrophobic nature. Such fillers are preferably present in an amount in the range of about 1% to about 20% by weight, based on the amount of polymeric material present in the gas sensing composition. A particularly useful filler is hydrophobic fumed silica, e.g., in the form of fine particles. The sensing component or components chosen for use are those effective to provide a signal which varies in response to variations in the analytes, for example, concentrations of the gas, of interest in the medium being monitored. The sensing component is preferably an optical indicator, such as fluorescence indicators and absorbance indicators, in particular fluorescence indicators. In sensing carbon dioxide concentrations, examples of absorbance indicators that can be used include chlorophenyl red, bromo cresol purple, nitrophenol, bromo thymol blue, penachlorome and pheno red. Useful fluorescence indicators for carbon dioxide include hydroxypyrene 3,6,8-trisulfonic acid, herein referred to as HPTS or hydroxypyrene trisulfonic acid, derivatives, e.g., salts, of HPTS, beta-methylumbelliferone or fluorescein. The more preferred sensing component, particularly for sensing the concentration of carbon dioxide in blood, is selected from HPTS, derivatives of HPTS and mixtures thereof. The alkali and alkaline earth metal salts of HPTS are useful HPTS derivatives. In oxygen sensing concentrations, examples of fluorescence indicators include one or more polynuclear aromatic compounds and derivatives of polynuclear aromatic compounds. Examples of such polynuclear aromatic compounds include decacyclene, benzo-ghi-perylene and coronene. The oxygen indicators may include a mixture of tertiary butyl derivatives of such polynuclear aromatic compounds. Such indicators are more fully described in Yafuso, et al U.S. Patent 4,849,172. The oxygen indicators may be covalently bonded to the polymeric materials or matrix materials included in the sensing composition, for example, so as not to be leachable into the medium being monitored. Such covalent bonding is preferably accomplished by providing an oxygen indicator component including a reactable group which reacts with a reactable group, preferably a different reactable group, present in one of the components of the precursor of the polymeric matrix material. Thus, during the formation of the polymeric matrix material, the above-noted reactable groups also react to covalently bond the oxygen indicator to the matrix material. Particularly useful oxygen indicator components include the above-noted polynuclear aromatic compounds derivatized to include a reactable group, such as a reactable group with functional carbon-carbon unsaturation. The vinyl derivatives of such compounds provide excellent results. The amount of sensing component used is such as to provide a sufficiently strong signal so that the analyte of interest can be reliably and accurately determined. The present sensing composition precursors and sensing compositions which include an aqueous liquid, such as those useful in sensing carbon dioxide, preferably also include an effective amount of at least one dispersing agent. Such agents act to facilitate maintaining the aqueous liquid dispersed in the sensing composition precursor prior to forming the polymeric material. Any suitable dispersing agent may be used provided that such agent has no substantial detrimental effect on the functioning of the present system or on the medium being monitored. Examples of dispersing agents include watersoluble dextran, polyvinyl alcohol, poly(ethylene oxide), polyvinylpyrrolidone and hydroxyalkyl celluloses. The amount of dispersing agent utilized may vary depending, for example, on the specific dispersing agent, polymer precursor and filler being employed. In one embodiment, the amount of dispersing agent present is in the range of about 1% to about 40% by weight of the total aqueous liquid employed. The precursor for the carbon dioxide sensing composition may be prepared as follows. The carbon dioxide sensing component is dissolved in a quantity of liquid water to form an aqueous liquid. If desired, a buffer and/or dispersing agent can also be dissolved in this aqueous liquid. The desired amount of polymer precursor, and, if desired, a filler component and polymerization catalyst, is (are) mixed with the aqueous liquid. One or more cross-linking agents may or may not be present at this point. The resulting mixture is subjected to strong agitation, for example, in a mechanical homogenizer, to disperse the aqueous liquid and form dispersed mixture. After this agitation, the dispersed mixture containing dispersed aqueous liquid can be stored for a period of time, e.g. on the order of about 1 hour to about 24 hours or more, ready for use. A cross-linking agent or agents, if any, and a polymerization catalyst, if not already present, are added to the dispersed mixture. These are gently stirred into the dispersed mixture to form the sensing composition precursor. After the sensing composition precursor has been prepared and degassed, a quantity of it is continuously placed, on one side of a continuous web. In one embodiment, the continuous web is transparent and/or is substantially impermeable to the analyte to be sensed. This precursor is preferably present on the continuous web as a continuous coating or sheet of substantially uniform thickness and composition. As noted previously, placing the precursor on the web continuously provides substantial sensing element manufacturing benefits and, ultimately, produces high quality sensing elements reliably and cost effectively. In one useful embodiment, the continuous placing of the precursor on the web is accomplished by continuously passing or moving the continuous web, preferably at a constant speed, in proximity to the stationary outlet of a coating apparatus which provides the precursor to the web through the outlet. Other coating methods effective to continuously place the sensing composition precursor on the continuous web may be employed. Many coating methods are conventional and well known in the coating art. Examples of such methods include roll coating; brush coating; knife coating, such as coating using a floating knife, air knife coating, knife-over-blanket coating and knife-over-roll coating; blade coating; rod coating; dip coating; squeeze roll coating; levelon coating; kiss coating; gravure coating; reverse roll coating; curtain coating; extrusion coating; slot-orifice coating; calender coating; precast coating; cast coating; spray coating; atomization; vacuum coating; and sputtering. Since the sensing composition precursor is continuously placed or coated on the continuous web, it is preferred that the coating method chosen be such that the web is in motion as it is being coated with precursor, more preferably from a stationary source of precursor. This "moving web" type of process provides easy and effective control of the coating operation so that substantially uniform coating of relatively long lengths of, for example, lengths of at least about 2 feet or at least about 5 feet to about 20 feet or about 50 feet or more, continuous web is achieved. The continuous web can be conveniently located on or in the form of a roll prior to being passed for coating. After the continuous sensing composition is formed from the precursor coating, the sensing composition-containing web can be placed on a roll or maintained in the form of a roll in anticipation of the next processing step. In addition, other intermediate products described herein can be placed on rolls or maintained as rolls of material in between processing steps. The precursor for the oxygen sensing composition may be prepared in a somewhat analogous manner to the carbon dioxide sensing composition precursor except that no aqueous liquid is used and an organic solvent is used to combine the sensing component into the polymer precursor. The invention involves the production of sensing elements each of which includes no portion of the continuous web, that is includes no web layer. Once the sensing composition-containing web is formed, the continuous web is removed from this sensing composition-containing web. Such removal may involve physically stripping the web from the continuous sensing composition, exposing the sensing composition-containing web to a selective solvent to dissolve the continuous web and/or one or more other web removing steps. Care should be exercised to avoid damaging the sensing composition in removing the web. For example, it may be advantageous to employ a water soluble web material, such as polyvinyl alcohol or the like, so that water can be used to remove the web without substantially detrimentally affecting the continuous sensing composition. Each of the individual sensing elements preferably includes opposing end surfaces, spaced apart by the thickness of the element, which are substantially mutually parallel. Each sensing element is preferably shaped to at least partially fit into a cavity or indent of a sensor holder or cassette, as described herein, preferably with one end surface facing the wall opposite the open end of the sensor holder cavity or indent. Each of the individual sensing elements produced in accordance with the present invention can, and preferably do, have the configuration of a right circular cylinder. In one embodiment, the web layer has a thickness in the range of about 0.0025 cm to about 0.254 cm, and preferably a diameter in the range of about 0.127 cm to about 1.27 cm. The individual sensing elements preferably have similar diameters. The sensing composition precursor located on the continuous web is converted or otherwise transformed into the continuous sensing composition. When, as is preferred, this sensing composition precursor includes a polymer precursor, this forming step comprises polymerizing, including cross-linking, the polymer precursor. This sensing composition forming step is preferably conducted at conditions effective to form the sensing composition without substantially detrimentally affecting the web or other components of the present system. Visible light energy or ultraviolet light energy and/or temperature, preferably in the range of about ambient temperature (22° C) or elevated temperature, as described herein, are often very effective in curing the preferred silicone polymer precursor to form a cross-linked silicone polymer-containing sensing composition. After the sensing composition precursor has been processed to form the continuous sensing composition, a continuous film of opaque material is preferably placed, for example, applied or formed, on the continuous sensing composition in such a manner that the sensing composition is located between the continuous web and the opaque film. This opaque film comprises the material from which the opaque layer of the present individual sensing elements is derived. The opaque film may be in the form of a porous membrane provided it has sufficient opacity and other properties to function as described herein as an opaque film. The opaque film and opaque layer are made of a material which is analyte, preferably gas, permeable. This opaque layer acts to provide a substantial degree of optical isolation for the sensing composition layer. This optical isolation facilitates the providing of a substantially focused or directed signal from the sensing component to be transmitted for analysis, as described herein. Ultimately, such optical isolation often results in increased accuracy and reliability of the concentration determinations obtained using the present sensing elements. The opaque layer should be freely permeable to the analyte of interest in the medium being monitored. This opaque layer analyte permeability allows the analyte of interest from the medium to contact the sensing composition and interact with the sensing component. The opaque layer is preferably made from a polymer combined with an opaque agent. Any suitable polymer may be included in the presently useful opaque layer, provided that the polymer has no substantial detrimental effect on the functioning of the present system or on the medium being monitored. The polymer chosen preferably provides a relatively thin film with sufficient structural integrity and durability to be useful in the present methods and sensing elements. In one embodiment, the opaque film includes a fluorine-containing polymer, preferably a polymer selected from polyfluorohydrocarbons, polyfluorocarbons and mixtures thereof, and especially, polytetrafluoroethylene. Although a pre-formed opaque film can be applied to the continuous sensing composition, in one particularly useful embodiment the opaque film is formed in situ , that is on the sensing composition. Thus, a mixture containing a precursor component of the polymer to be included in the opaque film and an opaque agent is applied to or placed (coated) on, preferably continuously placed on, the-sensing composition. This applied mixture is polymerized, for example, cured, to form the opaque film. This method of forming the opaque film provides substantial advantages, for example, providing an opaque film having a substantially uniform thickness which is strongly secured to the sensing composition. Employing continuous processing to apply the mixture to the sensing composition, as described herein, provides additional benefits many of which are similar or analogous to the benefits achieved by continuously placing the sensing composition precursor on the continuous web. The chemical make-up of the polymer included in the opaque film may be the same as or different from that of the polymeric matrix material of the sensing composition. In a very useful embodiment, the polymer included in the opaque film is a silicone-based polymer, preferably such a polymer with a similar, or even substantially the same, chemical make-up as the silicone-based polymeric matrix materials included in the sensing compositions. The use of silicone-based polymeric materials in the opaque film results in sensing elements and sensors which advantageously have more rapid response times. In one embodiment, the opaque film includes a vinyl/hydride addition cure polysiloxane polymer. A specific example of a useful polymer for inclusion in the opaque film is a polymer derived from vinyl terminated dimethyl siloxane, such as that described previously with regard to the polymeric matrix material of the sensing compositions. Opaque films including such polymers can be formed in situ on the sensing composition in substantially the same manner as described previously with regard to the formation of the sensing composition except that an effective amount of a suitable opaque agent is used in place of the sensing component. One exception is that photo-activatable catalysts are generally not used in producing the opaque film. Any suitable opaque agent may be used provided that such agent or agents function to provide the desired degree of optical isolation and have no ' substantial detrimental effect on the functioning of the present system or on the medium being monitored. Among the opaque agents useful in the present invention are carbon black, other carbon based opaque agents, ferric oxide, T i O 2 , BaSO 4 and metallic phthalocyanines. Such opaque agents are preferably substantially uniformly dispersed in the opaque layer in an amount effective to provide the desired degree of opacity, e.g., to provide the desired optical isolation. A particularly useful opaque agent is carbon black. Preferably, the opaque film is substantially thinner than the typical continuous web, for example, to facilitate analyte permeability. More preferably, the thickness of the opaque film is in the range of about 1% to about 20% of the thickness of the typical continuous web. In one embodiment, the opaque film has a thickness between 0.0001 cm to 0.005 cm, more preferably between 0.0003 cm to 0.0025 cm, most preferably between 0.0005 cm to 0.0015 cm. Like the web, the continuous opaque film preferably includes opposing end surfaces which are substantially mutually parallel. In one particularly useful embodiment, one of the end surfaces of the opaque film is structured to bond or adhere to the continuous sensing composition. This feature advantageously increases the integrity of the composite structures and individual sensing elements in accordance with the present invention. Such end surface in contact with the continuous sensing composition may inherently be structured to provide this enhanced opaque film/sensing composition bond strength, or it (and/or the sensing composition) can be treated, for example, by exposure to plasma and/or corona discharge and/or by being coated with a primer agent and/or coupling component and/or by being subjected to other surface modification techniques, so as to provide this enhanced bond strength. Pressure, such a laminating pressure, can be applied to the transparent web/sensing composition/opaque film composite to obtain enhanced bond strength between the components of the composite. Once this sensing composition/opaque film composite structure is formed, random samples of the composite can be removed and tested or screened, using conventional techniques, to determine if the composite structure meets the product specifications for sensing elements in the desired application. If such testing or screening determines that the composite structure is not within product specifications, it can be discarded and the process started again. However, if the results of the random testing or screening are positive, that is that the composite structure does meet product specifications, no further testing may be needed prior to using individual sensing elements derived from this composite structure in a sensing application. If this composite structure is rolled up, it may be desirable to have a non-adhesive layer or sheet between the layers of composite on the roll. This non-adhesive sheet prevents the composite layers on the roll from sticking together. Assuming that a composite structure meeting product specifications has been produced, the structure is then processed, for example, using a punch, a "cookie cutter-type" device or the like cutting device to form individual sensing elements, which are then available for being secured to a sensor holder or fixture for use. For example, the individual sensing elements can be placed at least partially in an open ended cavity in a sensor cassette. The sensing element is secured, for example, by adhesive, to the sensor holder. Any suitable, preferably transparent, cure-in-place adhesive composition may be employed, provided that such compositions have no substantial detrimental effect on the present system or on the medium being monitored. Examples include silicone-based adhesives, two-part urethane adhesives and epoxy adhesives. As previously mentioned, the sensing composition itself can also perform the function of the adhesive member and stick or adhere to the sensor holder. Alternately, the sensing element can be secured to the sensor holder or fixture mechanically, such as through the use of clamps or other mechanical securement assemblies, or can be press fitted in place on the sensor holder or fixture. Other approaches may be employed to secure the sensing element to the sensor holder or fixture. The sensing element is situated relative to the sensor holder so that the opaque layer is exposed to the medium being monitored. Sensing elements which include no opaque layer may be produced and utilized in accordance with the present invention. The sensor holder is preferably placed in proximity to an excitation assembly positioned and adapted to provide an excitation signal to the sensing element. A detector assembly is positioned and adapted to detect an emitted signal from the sensing element, which is capable of providing the emitted signal in response to being exposed to the excitation signal. A processor assembly is positioned and adapted to analyze the emitted signal (or a signal corresponding to the emitted signal) in determining the concentration of the analyte of interest in the system being monitored. When sensing carbon dioxide, a particularly useful sensing composition comprises a two phase system comprised of a pH indicator containing an aqueous phase encapsulated in a non-polar, CO 2 permeable barrier material. Unfortunately a characteristic feature of these types of sensors is reversible "CO 2 conditioning drift", a response instability accentuated by large changes in CO 2 partial pressure. We have discovered an optical fluorescence based sensor (e.g., for measuring CO 2 concentration in a medium such as blood) with improved drift stability. We have discovered that adventitious pH-titratable partitioning species other than the analyte of interest can reversibly migrate between the indicator and barrier phases as a function of pH, and therefore as a function of CO 2 partial pressure. We have also discovered that these species can also irreversibly migrate (or "leach") from the sensor to the medium. The migration of the partitioning species generates a pH response which can substantially affect the analyte concentration dependent signal. By careful choice and/or purification of sensor materials and components, we have minimized the presence of these adventitious species and developed a substantially drift-free sensor formulation. Alternatively, by proper choice of buffer composition and indicator pKa, we have discovered compositions which minimize the CO 2 dependent migration of adventitious species, further stabilizing these sensors. We have made the unexpected discovery that one cause of the drift instability (commonly referred to as "analyte conditioning drift" or more specifically as "CO 2 conditioning drift") is actually a pH hysteresis phenomenon resulting from the presence of pH-titratable materials (hereinafter referred to as "partitioning species") which migrate in and out of the aqueous indicator phase as a function of internal pH, and therefore as a function of CO 2 partial pressure. While ionized species are essentially insoluble in non-polar barrier materials such as silicone, the neutral form of organic acids or bases can be very soluble in both the aqueous and silicone phases. For example, if an air equilibrated sensor comprising a sodium acetate impurity in the aqueous indicator phase is suddenly exposed to an elevated CO 2 level, such that the internal compartment pH drops from pH 9 to pH 7, a sudden 100 fold increase in acetic acid concentration occurs creating a thermodynamic driving force for slow migration of charge neutral acid into the silicone. Acid depletion from the aqueous indicator phase then induces additional CO 2 uptake and further protonation of acetate ions. This replacement of sodium acetate by sodium bicarbonate changes the pH/pCO 2 relationship for the aqueous indicator phase, resulting in a slow rise in pH in opposition to the initially imposed pH drop. The migration or "partitioning" process continues until the equilibrium partitioning ratio for acetic acid is reestablished between the aqueous indicator phase and the silicone. Upon returning to the air equilibrated baseline the process is reversed. Although not recognizing the cause of the CO 2 drift problem, it has been a customary practice in the sensor field to package continuous blood gas monitoring sensors in a "CO 2 conditioned state" (i.e., at elevated CO 2 levels) thereby somewhat reducing excessive drift upon initial exposure to physiological CO 2 levels. With our new formulation, this practice should no longer be required. A corresponding hysteresis process is operative for organic bases such as amines, since they can also migrate from the silicone phase into the aqueous indicator phase when the CO 2 level is elevated. For example, if an air equilibrated sensor comprising an amine impurity in the silicone phase is suddenly exposed to an elevated CO 2 level, such that the internal compartment pH drops from pH 9 to pH 7, a sudden 100 fold increase in ammonium ion concentration occurs creating a thermodynamic driving force for slow migration of charge neutral amine from the silicone into the aqueous phase. Amine uptake to the aqueous indicator phase then induces additional CO 2 uptake and further protonation of amine. This accumulation of ammonium ions changes the pH/pCO 2 relationship for the aqueous indicator phase, resulting in a slow rise in pH in opposition to the initially imposed pH drop. The migration or "partitioning" process continues until the equilibrium partitioning ratio for amine is reestablished. Upon returning to the air equilibrated baseline the process is reversed. Notably, the effect on the sensor response is identical for both acetate depletion and amine uptake; that is, when moved from an air equilibrated medium to a medium equilibrated at a higher CO 2 level, a negative mm drift will result in the presence of either type of species. This hysteresis process can also occur for the indicator dye itself if the dye exists in equilibrium with a partitionable charge neutral form. Another cause of drift instability (referred to as "saline conditioning drift" or "saline drift") is actually a pH dependent phenomenon resulting from the presence of pH-titratable materials which migrate from the sensor to the medium as a function of external pH, and therefore as a function of CO 2 partial pressure. For example, if an air equilibrated sensor comprising an amine impurity in the silicone is suddenly exposed to an elevated CO 2 level, the pH of the external buffer drops from pH 9 to pH 7, creating a driving force for slow migration of charge neutral amine from the silicone into the external aqueous medium where it becomes protonated. This process impacts the amount of amine available for partitioning into the internal aqueous compartment. Notably, the effect on the sensor response is opposite to the previously discussed CO 2 conditioning drift. That is to say, the sensor exhibits a positive mm drift due to these migrating species. With this new understanding, we have developed new CO 2 sensor formulations which contain less than a critical amount of titratable partitioning species, and exhibit substantially drift free response. As used herein the terms "drift free" or "substantially drift free" mean that the sensor provides a signal which drifts less than 12 % (i.e., < 5.5 mm) over a three hour period when moved from a medium equilibrated with a gas having a pCO 2 of 0.25 mm to a medium equilibrated with a gas having a pCO 2 of 45.6 mm as herein described (see Example 1 and FIG. 3b for details of this calculation). More preferably, the sensor provides a signal which drifts less than 6 % when moved from a medium equilibrated with a gas having a pCO 2 of 0.25 mm (i.e., air equilibrated) to a medium equilibrated with a gas having a pCO 2 of 45.6 mm. Most preferably, the sensor provides a signal which drifts less than 3 % when moved from a medium equilibrated with a gas having a pCO 2 of 0.25 mm to a medium equilibrated with a gas having a pCO 2 of 45.6 mm. This advance offers several practical advantages. In one embodiment, the sensor is maintained in an air-equilibrated buffer condition, except during temporal physiological CO 2 sampling. For example, certain commercial ex-vivo sensing systems contain a sensing element (e.g., a sensor composition housed in a cassette which is attached to an optical fiber or which alternatively contains an excitation light source) in the a-line circuit of arterially catheterized patients. Blood gas levels are monitored temporally (e.g., "on demand" by the health care provider or automatically by means of a programmed pump or motor) by drawing blood up the saline drip line into the sensor cassette, and then allowing the blood to return to the patient. The present invention, when used in such a configuration, should remove earlier limitations on the frequency and duration of physiological CO 2 sampling that can be achieved without incurring or inducing drift. Sensors of the present invention are easily calibrated and may reside in the calibration medium before, during and after calibration without the special precautions commonly employed to limit "CO 2 conditioning drift" or "saline drift". In general, the magnitude of drift exhibited by traditional sensors is proportional with the buffering capacity of the aqueous indicator compartment. Sensors of the present invention, preferably being substantially free of partitioning species, enjoy the advantage of being far less susceptible to drift even when the buffering capacity of the aqueous indicator phase is decreased. This enables design of sensors with a faster response time, without introducing a prohibitive rate and/or amount of drift. In addition, the gas sensors of the invention are stable (i.e., drift free), reproducible and tolerant of production variables without detracting from the inherent properties of the gas sensors. Alternatively, one can reduce the drift exhibited by sensors that contain small amounts of partitioning species by employing higher buffer concentrations as herein described. Finally, one may combine a higher buffer concentration with a "clean" sensor chemistry and produce a drift free sensor which is less susceptible to externally induced drift such as might be caused by exposure of the sensor to a medium which contains a partitioning species. This can be advantageously accomplished in a gas sensor which comprises an aqueous first phase including a dye and a hydrophobic second phase, and which is substantially free of partitioning impurities which can migrate from one phase to the other in response to a change in pH in the first phase and which substantially affect the concentration dependent signal. In a preferred embodiment, the first phase comprises an aqueous buffered solvent and a dye and the second phase comprises a cross-linked polymeric material which is gas permeable, light permeable and essentially aqueous impermeable. In a presently preferred embodiment, the first and second phases are formed into a permanent "emulsoid" of suspended or dispersed micro-compartments of the aqueous first phase in the cross-linked polymeric second phase wherein the micro-compartments of the aqueous first phase are preferably smaller than 5 microns, and more preferably smaller than 2 microns. In an illustrative embodiment of the invention, the dye is a pH sensitive dye, the aqueous buffer solvent is a physiological pH range buffer solution as for instance a bicarbonate ion or bicarbonate/phosphate ion based buffer solution. In this illustrative embodiment the polymeric material is a silicone material as for instance a siloxane material which is carbon dioxide permeable. More specifically the material is polydimethylsiloxane or polydimethylsiloxane copolymers. The dye in the illustrative embodiment is the trisodium salt of hydroxypyrene trisulfonic acid ("HPTS"). In an illustrative embodiment of the process, the dye is present in the aqueous phase in a concentration of about 1 to about 15 millimolar and the buffer is present in the aqueous phase in a concentration of from about 1 to about 100 millimolar. More preferably, the dye and buffer are present in a concentration of from about 1 to about 10 and 1 to 50 millimolar, respectfully, and most preferably the dye and buffer are present in a concentration of from about 1 to 5 and 5 to 20 millimolar, respectfully. A further advantageous process of preparing a gas sensor comprises dissolving a quantity of a dye in a quantity of aqueous buffer solution followed by vigorously mixing the buffer solution with a quantity of a polymeric precursor of a cross-linked polymeric material so as to form an emulsion (or suspension) of the buffer solution and the polymeric precursor. Then a quantity of a cross-linking agent and catalyst are added to the emulsion. The catalyzed emulsion is formed into a shape (e.g., coated into a sheet, as herein described, suitable for insertion into a sensor cassette) and cured to form a permanent emulsoid (which is substantially free of partitioning species which can migrate from one phase to the other in response to a change in pH in the buffer solution and which substantially affect the analyte concentration dependent signal) of micro-compartments of the dye containing aqueous buffer solution in the cross-linked polymeric material. The above process can be augmented by adding a quantity of an emulsification enhancement agent (e.g., a thickener or surfactant) to the solution of the dye in the buffer so as to form a mixture of the dye and the emulsification enhancement agent in the buffer and/or by adding a dispersing agent such as fumed silica to the hydrophobic phase. The following non-limiting examples illustrate certain aspects of the present invention. Unless otherwise indicated, all parts and percentages are by weight. EXAMPLES Example 1 (Reference Example) An aqueous solution is formed by dissolving 11.8 mg of hydroxypyrenetrisulfonate (trisodium salt), 375 mg of poly (ethylene oxide) having a molecular weight of 300,000, 31.4 mg of trisodium phosphate dodecahydrate, and 59.6 mg of sodium chloride in enough water to make 7.5 gm of solution. A silicone mixture is prepared by combining 16.4 mg of cyclopentadienyl trimethyl platinum (an ultraviolet light activated catalyst component), 0.49 gm of fumed silica, sold by Cabot under the trademark TS-530, and 15.91 gm of vinyl dimethyl end-capped poly(dimethyl siloxane) having a viscosity of 1000 centistokes and sold by NuSil Silicone Technology under the trademark PLY-7500. The aqueous solution is combined with the silicone mixture, and 1.1 gm of poly(dimethyl) (methylhydro)-siloxane, sold by NuSil Silicone Technology under the trademark XL-123, having a molecular weight of about 2100 and a silyl hydride group content of about 30%, is added. This combination is then processed with a Vertis Cyclone IQ homogenizer to form the sensing composition precursor. A transparent, continuous polycarbonate web, having a thickness of about 0.0127 cm, is coated with an adhesion enhancement component derived from a mixture containing water, 1.25% by weight of colloidal silica particles, 0.11% by weight of aminopropyltriethoxysilane, 0.05% by weight ammonium hydroxide and 0.03% by weight of a surfactant sold by Rohm and Haas under the trademark Triton X-100. Using a coating apparatus sold by Hirano under the trademark M-200, the sensing composition precursor is continuously coated on one side of the web. In this coating operation (and all other coating operations described herein using this coating apparatus), the web is moved, at a constant speed, in proximity to and under the coating apparatus which is stationary. This precursor coating has a substantially uniform thickness of about 0.0025 cm. The sensing composition precursor is cured by exposing the coating to about 70 mJ cm -2 of ultraviolet (365 nm)light. The coating is further cured by exposure at about 90° C for about 2 minutes to form the sensing composition. An opaque film precursor is prepared by mixing 12 gm of a 12% by weight dispersion of carbon black, sold by Cabot under the trademark Regal 99R, in vinyl end-capped poly(dimethyl)siloxane sold by NuSil Silicone Technology under the trademark PLY-7501, having a viscosity of 500 centistokes, with 26.6 mg of a platinum catalyst solution sold by NuSil Silicone Technology under the trademark Cat50, 6.5 mg of a polymerization inhibitor sold by Nusil Silicone Technology under the trademark XL119, and 0.6 gm of the poly(dimethyl)(methylhydro)siloxane noted above. Using the coating apparatus noted above, this opaque precursor is continuously coated on the sensing layer in a substantially uniform coating with a thickness of about 0.0013 cm. The opaque film precursor is cured by exposure at 70° C for 2 minutes to form the opaque film. Individual carbon dioxide sensing elements are cut from the continuous web/sensing composition/opaque film composite. Each sensing element includes a generally circularly shaped portion of the transparent web, a generally circularly shaped portion of an opaque film and a thin, generally circularly shaped layer of the sensing composition therebetween. Such an individual sensing element is illustrated in Figs. 2 and 3. Using conventional testing procedures, a small, representative sample of the individual carbon dioxide sensing elements is tested to determine if such elements meet product specifications, that is if such elements are effective for accurately and reliably sensing the concentration of carbon dioxide in human blood. These individual carbon dioxide sensing elements are found to be satisfactory based on this testing procedure. Using a transparent silicone-based adhesive sold by Dow Corning under the trademark Dow Corning 3140, one of these carbon dioxide sensing elements is bonded into a right circular cylindrical open ended well, having a diameter of 0.317 cm and a depth of 0.025 ± 0.0025 cm, formed in a polycarbonate cassette so that the transparent web layer is facing the bottom of the well. Prior to bonding the sensing elements to the cassette, the walls of the well are contacted with a priming agent, sold by Dow Chemical Company under the trademark Dow 1205, to promote adhesion between the sensing element and the polycarbonate cassette. The thus produced carbon dioxide sensor is effective in determining the concentration of carbon dioxide in blood brought into contact with the opaque layer. Example 2 (Reference Example) Example 1 is repeated except that after the opaque film is formed the composite is processed as follows. A release liner of a polyester film coated with perfluoro polyether and sold by Minnesota Mining and Manufacturing Company under the trademark Scotch Pack 1022 is coated on one side with a pressure sensitive adhesive composition sold by General Electric under the trademark PSA-518. This coated sheet is applied to the side of the transparent web opposite the sensing composition so that the adhesive is in contact with the transparent web. This formed composite is then subjected to a cutting operation such that generally right circular cylindrical sensing elements are formed and remain located on the release liner which remains intact during this curing operation. Each of the sensing elements includes an opaque layer, a sensing composition layer, a transparent web layer and an adhesive composition layer. Such a sensing element is illustrated in Fig. 4. After the cutting operation, the material which is located on the release liner which is not part of any sensing element is removed or weeded out from the release liner. This non-sensing element material is discarded. A small, representative sample of the sensing elements located on the release liner are removed and tested, using conventional techniques, to determine whether or not the sensing elements on the release liner meet product specifications. Once it has been determined that these sensing elements are satisfactory, they can be used simply by removing them from the release liner and placing them in an open ended cavity of a sensor cassette so that the adhesive composition layer is in contact with the bottom of the sensor holder cavity. By pressing the sensing element into the cavity, the adhesive composition acts to securely adhere the sensing element to the sensor cassette. The thus produced carbon dioxide sensor is effective in determining the concentration of carbon dioxide in blood brought into contact with the opaque layer. Example 3 (Reference Example) A 0.457 gm portion of 4-vinylbenzo[g,h,i]perylene is mixed with 10.0 gm of poly(methylhydro)(dimethyl)-siloxane described in Example 1 and 300 ml toluene. A 100 microliter portion of a platinum catalyst solution sold by Huls America under the trademark PC 072 is added, and the solution is heated to reflux for 1 hour, then cooled to room temperature. A 8 gm portion of decolorizing charcoal is added. After stirring, the charcoal is removed by filtration. This step removes the platinum catalyst. The toluene is removed under vacuum, and the residue made up to 100 ml with hexane. A 0.7 mg portion of cyclopentadienyl trimethylplatinum is dissolved in 7.0 gm of poly(dimethyl)siloxane having a viscosity of 500 centistokes. To this solution is added 1.9 ml of the hexane solution described above. The hexane is removed under vacuum to form the sensing layer precursor. A transparent poly(ethylene terephthlate) web, having a thickness of 0.0127 cm, is coated with the adhesion enhancement component derived from the mixture described in Example 1. Using the coating apparatus identified in Example 1, this sensing composition precursor is continuously coated on one side of the web. This precursor coating has a substantially uniform thickness of about 0.0013 cm. The sensing composition precursor is cured by exposing the coating to about 70 mJ cm -2 of ultraviolet (365 nm)light. The coating is further cured by exposure at about 90° C for about 2 minutes. An opaque film precursor is prepared as described in Example 1. Using the coating apparatus identified in Example 1, this opaque precursor is continuously coated on the sensing layer in a substantially uniform coating, with a thickness of about 0.0008 cm. The opaque precursor is cured by exposure at 70° C for 2 minutes. Individual oxygen sensing elements are cut from the transparent web/sensing composition/opaque film composite. Each sensing element includes a generally circularly shaped portion of the transparent web, a generally circularly shaped portion of an opaque film and a thin, generally circularly shaped layer of the sensing composition therebetween. Using conventional testing procedures, a small, representative sample of the individual oxygen sensing elements is tested to determine if such elements meet product specifications, that is if such elements are effective for accurately and reliably sensing the concentration of oxygen in human blood. These individual oxygen sensing elements are found to be satisfactory based on this testing procedure. Using a transparent, room temperature vulcanizable silicone-based adhesive sold by General Electric under the trademark RTV 128, as sensing element, as noted above, is bonded into a right circular cylindrical open ended well, having a diameter of 0.3175 cm and a depth of 0.025 ± 0.0025 cm, formed in a polycarbonate cassette so that the transparent web layer is facing the bottom of the wall. Prior to bonding the sensing element to the cassette, the walls of the well are contacted with a priming agent, sold by Dow Chemical Company under the trademark Dow 1205, to promote adhesion between the sensing element and the polycarbonate cassette. The thus produced oxygen sensor is effective in determining the concentration of oxygen in blood brought into contact with the opaque layer. Example 4 (Reference Exmaple) Example 2 is repeated except that the continuous web/sensing composition/opaque film composite of Example 3 is employed. Sensors, including individual oxygen sensing elements, are produced and are effective in sensing the oxygen concentration of blood brought into contact with the opaque layer. Figs. 2 and 3 illustrate the use of a sensing element produced in Example 1 in determining carbon dioxide concentrations. A sensing element produced in Example 3 can be used in a substantially similar manner in determining oxygen concentrations. As shown in Figs. 2 and 3, this individual sensing element 50 is placed into well 54 of sensor holder 56 and bonded in place using a layer 57 of transparent, silicone-based adhesive. Well 54 is open at one end, includes a right circular cylindrical side wall 55 and a circular bottom end wall 58. The size of well 54 is such that the individual sensing element 50 and silicone-based adhesive layer 57 completely fill the well. Individual sensing element 50 is placed in well 54 so that the transparent web layer 64 faces the bottom end wall 58 of well 54. The opaque layer 62 includes an exposed surface 63 (Fig. 3) which is raised relative to the inner surface 70 of sensor holder 66. The opaque layer 62 substantially shields sensing composition layer 66 from direct contact with the medium, e.g., blood, to be monitored. Depending on the specific sensing application involved, the exposed surface of the opaque layer can be recessed relative to, or flush with, the inner surface of the sensor holder. Referring now to Fig. 3, in use sensor holder 56, made of a transparent polycarbonate material, is placed in abutting relation to optical fiber 72. Optical fiber 72 provides excitation light of appropriate wavelength from light transmitting apparatus 74 to excite the sensing component in the sensing composition layer 66 to fluoresce and provide a signal characteristic of the concentration of carbon dioxide located in the medium in contact with the opaque film 62. This optical fiber 72 also transmits the signal which is emitted from the sensing component and passes such signal to a light detecting or receiving apparatus 76, from which a signal corresponding to this emitted signal is passed to a conventional electronic processor device 77 which processes or analyzes this emitted signal (the corresponding signal), e.g., as described in Lubbers et al U.S. Reissue Patent No. 31,879 and Heitzmann U.S. Patent 4,557,900, in determining the concentration of carbon dioxide in this medium. Over a period of time the individual sensing element 50 provides consistent signals which are reliably correlated to the true and accurate concentration of carbon dioxide in the blood in contact with the opaque layer 62. Fig. 4 illustrates the use of a sensing element produced in Example 2 in determining carbon dioxide concentrations. A sensing element produced in Example 4 can be used in a substantially similar manner in determining oxygen concentrations. As shown in Fig. 4, this individual sensing element 80 is placed into well 54 of sensor holder 86 and is bonded in place using pressure sensitive adhesive layer 87, which is a component of the individual sensing element. Well 54 is open at one end, includes a right circular cylindrical side wall 85 and a circular bottom end wall 88. The size of the well 54 is such that the individual sensing element 80 completely fills the well. Individual gas sensing element 80 is placed in well 54 so that the transparent web layer 84 faces the bottom end wall 88 of well 54. The opaque layer 82 includes an exposed surface 89 which is raised relative to the inner surface 90 of sensor holder 86. The opaque layer 82 substantially shields sensing composition layer 86 from direct contact with the medium, e.g., blood, to be monitored. Depending on the specific sensing application involved, the exposed surface of the opaque layer can be recessed relative to, or flush with, the inner surface of the sensor holder. Once the individual sensing element 80 is secured in well 54 of sensor holder 86, by manually pressing the sensing element into the well, it is employed in a manner analogous to the individual sensing element 50, as illustrated in Fig. 3. Over a period of time, the sensing element 80 provides signals which are reliably correlated to the true and accurate concentration of carbon dioxide in the blood in contact with the opaque layer 82. The present invention provides substantial benefits, for example, in terms of labor saving, cost saving and sensor performance. The present methods of mass producing sensing elements reduce the amount of labor and other resources needed to produce high quality sensors. The present methods provide for producing individual sensing elements which have substantially uniform characteristics, and avoid the variability inherently present in making individual sensing elements one by one. Because of the substantial uniformity achieved among the individual sensing elements produced, only limited testing or screening is necessary in order to determine that all of the sensing elements produced meet product specifications. The sensing elements produced in accordance with the present invention have outstanding sensing characteristics and are provided in a form which is very convenient for inclusion in a sensor holder or fixture. Thus, the present invention advantageously makes sensing an analyte of interest, for example, a gas, in a medium, for example blood, more cost effective, convenient, and reliable. Example 5 A 0.37 gm portion of a vinyl functional silicone (General Electric 1145-122) was mixed with a 0.45 gm portion of a silicon-hydride functional silicone (General Electric 1145-124). To this mixture was added 0.014 gm of a crosslinker/dye compound consisting of silyl-hydride covalently bonded to isobenzperylene. The formulation was thoroughly mixed to form a hydrosilylation cure silicone PSA and cast onto a release liner. The cast PSA sensing composition was 0.0051 cm thick. The cast film was held at 110 °C for 30 minutes at the end of which time a dry, tacky, uniform film resulted. To the above PSA/sensor film was laminated a 0.0051 cm thick black teflon film. Individual sensing elements of 2 mm diameter were die cut from the laminate. The release liner was removed from the teflon/PSA sensor/release liner laminate, and the individual sensing element was placed into the well of a polycarbonate sensor holder. The sensing element adhered strongly to both the teflon layer and the polycarbonate sensor holder, and could be removed only with difficulty even after 45 days aging. To demonstrate the ability of the sensing element to sense oxygen, a one square cm piece of the aforementioned laminate was placed into a gas flow-through cell and tested when exposed to a 0 % oxygen (i.e., pure nitrogen) and a 10% oxygen atmosphere. Using a 390 nm excitation signal (generated by a Spex FluoroLog-2 fluorimeter), emission intensities at 420 nm were recorded. For the above sensing element the emission intensity at 0% O 2 was 1.98x10 6 , and at 10% O 2 was 1.33x10 6 . A slope of 0.007 mm -1 was calculated for this sensing element which compares favorably to slopes found in typical non-adhesive O 2 sensors. The above data illustrate that an adhesive sensing element can be fabricated. The PSA matrix is comprised of a crosslinked poly(dimethylsiloxane) and a tackifying resin. The PSA matrix bonds securely to both a sensor holder and an optical isolation barrier film. This provides a sensing element with very simple construction and eliminates the need for special coatings or treatments used to enhance the adhesion of traditional silicone matrices to sensor holders and/or optical barrier films. While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and it can be variously practiced within the scope of the following claims.

Claims: A method for making a sensor capable of sensing an analyte, which method comprises: continuously placing a sensing composition precursor on a continuous web thereby forming a continuous precursor-containing web; forming a continuous sensing composition responsive to the analyte from said sensing composition precursor included in said continuous precursor-containing web, thereby producing a sensing composition-containing web, said continuous sensing composition being sized and structured to be dividable among a plurality of sensing elements; and wherein said forming step includes removing said continuous web from said sensing composition-containing web. A method for making a sensor capable of sensing an analyte according to claim 1, which method further comprises: placing a continuous opaque layer or film on said continuous sensing composition, thereby forming a composite structure. A method for making a sensor capable of sensing an analyte according to any of claims 1 or 2, wherein the sensing composition precursor comprises a sensing component, a polymer precursor, and an effective amount of a light activatalbe polymerization catalyst component; and the continuous sensing composition responsive to the analyte is formed from said sensing composition precursor by exposing said sensing composition precursor to sufficient light energy to activate said light activatable polymerization catalyst component and to partially polymerize said polymer precursor, and exposing said partially polymerized polymer precursor to elevated temperature to further polymerize said partially polymerized polymer precursor. The method of any preceding claim, wherein said continuous sensing composition is present as a layer having a substantially uniform thickness on said continuous web and is useful in sensing the concentration of a gas in a medium. The method of any preceding claim, wherein said continuous web comprises a transparent material which is polymeric. The method according to any of claims 1 to 5, wherein said sensing composition precursor comprises a sensing component, a polymer precursor and an effective amount of a polymerization catalyst component, and said forming step comprises polymerizing said polymer precursor and includes exposing said sensing composition precursor to elevated temperature to facilitate said polymerization catalyst component in promoting polymerization. The method of any preceding claim, which further comprises placing a continuous opaque film on said sensing composition, said continuous opaque film being permeable to the analyte. The method of any preceding claim, which further comprises forming a plurality of sensing elements from said sensing composition-containing web, each of said plurality of sensing elements including a sensing composition layer, and securing an individual sensing element formed from said sensing composition-containing web to a sensor fixture. The method of any preceding claim, wherein said sensing composition comprises a pressure sensitive adhesive capable of securely adhering to a sensor holder and to said continuous web. The method of any preceding claim, wherein said continuous sensing composition is useful in sensing the concentration of a gas in a medium, wherein said opaque film is formed by polymerizing an opaque film precursor comprising an opaque agent and a polymeric material precursor, and said continuous sensing composition and said continuous opaque film are present as layers each having a substantially uniform thickness. The method of any preceding claim, which further comprises forming a plurality of sensing elements from said composite structure, each of said plurality of sensing elements including a sensing composition layer and an opaque film layer, and securing one of said sensing elements to a sensor fixture. The method of any preceding claim, wherein said continuous web comprises polycarbonate, wherein said analyte is carbon dioxide, and wherein said pH sensitive indicator component comprises HPTS. A sensor for measuring the concentration of an analyte in a medium comprising a sensing element obtainable by the method according to any of claims 1 to 12. A sensor for measuring the concentration of an analyte in a medium comprising: a sensing element obtainable by the method according to any of claims 1 to 12 an excitation assembly positioned and adapted to provide an excitation signal to said sensing element; a detector assembly positioned and adapted to detect an emitted signal from said sensing element, said sensing element being capable of providing said emitted signal in response to being exposed to said excitation signal; and a processor assembly positioned and adapted to analyze said emitted signal in determining the concentration of said analyte in said medium. A sensor comprising: a sensor fixture including a surface adapted to be exposed to a medium containing an analyte the concentration of which is to be determined; and a sensing element obtainable by the method according to claims 1 to 12 secured to said sensor fixture, said sensing element being located relative to said surface so as to be exposed to said medium when said surface is so exposed. The sensor of claim 15, wherein said sensor fixture includes a cavity and said sensing element is at least partially located in said cavity. A method for making a sensor capable of sensing an anslyte, which method comprises: continuously placing a sensing composition precursor on a continuous web thereby forming a continuous precursor-containing web; forming a continuous sensing composition responsive to the analyte from said sensing composition precursor included in said continuous precursor-containing web, thereby producing a sensing composition-containing web, said continuous sensing composition being sized and structured to be dividable among a plurality of sensing elements; and forming a plurality of sensing elements from said sensing composition-containing web, each of said plurality of sensing elements including a sensing composition layer, and securing an individual sensing element formed from said sensing composition-containing web to a sensor fixture; wherein each of said plurality of sensing elements includes no portion of said continuous web.

Inventors: BENTSEN JAMES G, BRETSCHER KATHRYN R, DEKTAR JOHN L, PRATT RONNIE L, WOOD KENNETH B, YAFUSO MASAO, BENTSEN, JAMES G., BRETSCHER, KATHRYN R., DEKTAR, JOHN L., PRATT, RONNIE L., WOOD, KENNETH B., YAFUSO, MASAO, BENTSEN, JAMES G., C/O MINNESOTA MINING AND, BRETSCHER, KATHRYN R., C/O MINNESOTA MINING AND, DEKTAR, JOHN L., C/O MINNESOTA MINING AND, PRATT, RONNIE L., C/O MINNESOTA MINING AND, WOOD, KENNETH B., C/O MINNESOTA MINING AND, YAFUSO, MASAO, C/O MINNESOTA MINING AND
Applicants: TERUMO CARDIOVASCULAR SYS, TERUMO CARDIOVASCULAR SYSTEMS CORPORATION
Application year: 1994
Main classification: G01N33
IPC codes: G01N33, G01N31